1. Field of the Invention
The present invention relates generally to a reproduced color correction system for correcting a reproduced color of a printed matter, and more particularly to a reproduced color correction system for correcting a reproduced color of a printed matter so that a perceptual color of the printed matter may be the same even under illuminant with different color temperatures or non-standard illuminant.
2. Description of the Related Art
In the prior art, there has been a technical object to exactly reproduce a color of a printed matter under a single standard illuminant such as D50 or D65. In other words, conventionally, color evaluation or color matching of a printed matter is performed under a single standard illuminant.
On the other hand, a color of a printed matter observed under a specific standard illuminant is observed as a different color under non-standard illuminant or other standard illuminant. In the prior art, however, it has not been required so much to evaluate a color under an illuminant different from a specific light source. Thus, there has been no technical object to perform color matching so that a color of a printed matter may be the same even if a reference standard illuminant for observation is changed. At present, few techniques have been developed to correct a color of a printed matter so that the color may be observed as the same even under various standard illuminants, and not under a single standard illuminant.
With recent development of simplified color printing machines such as a color printer in color desk top publishing (DTP) color printed matters have been used for various purposes. Specifically, it has been required that color printed matters be observed under various viewing condition. At the same time, it has been required that a reproduced color be corrected when viewing condition are changed, so that a perceptual color of a printed matter may be the same.
At present, it is necessary not only to perform color matching under specific illuminant, but also to correct a color of a printed matter when the specific illuminant is optionally changed, so that it may be observed as the same as before the change of the illuminant.
In an example of a conventional color transformation technique for color matching, a neural network has been utilized. By making use of the neural network, a desired color is reproduced with high accuracy under a specific illuminant. For example, even when colors are reproduced by color printers with different chemical characteristic of the colorant or mechanical characteristic of the pinter, matters of the same colors can be obtained. A procedure of using the neural network in this case will be described below. By way of example, a multilayered feed-forward type neural network having a learning function is used.
FIG. 1 shows an example of a color transformation technique using the neural network. As is shown in FIG. 1, for example, coordinate values (or colorimetric values) 11 in a uniform color space such as "L*a*b*" or "L*u*v*" are provided. These coordinate values are set with a specific standard illuminant used as a reference illuminant. The "L*a*b*" or "L*u*v*" is a color system specified by the Commission Internationale de I'Eclairage (CIE). Coordinate values 12 are output as electric signals capable of being processed by a computer. On the other hand, the neural network 1 has a learning function. By using the learning function, desired color transformation can be realized. The neural network 1 is prepared for a specific standard illuminant, and the training of the neural network is performed with respect to the specific illuminant. The coordinate values 11 are converted to the coordinate values 12 of "RGB", "CMY", "CMYK", etc. by the neural network 1.
At the time of learning, some samples colors, of which pre-transformation coordinate values 11 and corresponding coordinate values 12 are known in advance, are used. At first, pre-transformation coordinate values, the post-transformation coordinate values of which are already known, are given to an input layer 1a of the neural network. Subsequently, the post-transformation coordinate values are given to an output layer 1c of the neural network as teacher signals. On the basis of these given coordinate values, the learning is performed for some sample colors. After completion of the learning, the connection weight among neurons is determined. Thus, the coordinate values 11 are converted to the coordinate values 12 by the neural network which has completed the learning. Based on the coordinate values 12, a desired reproduced color is obtained.
As has been described above, by using the neural network, a more exact reproduced color can be obtained under a specific standard illuminant.
Another example of the color transformation technique is a look-up table method. As is shown in FIG. 2, a three-dimensional input/output space is provided. Each of the three axes defining the input/output space is equally divided into several regions. Specifically, four regions are made as shown in FIG. 2. Thus, in the input/output space, 5.times.5.times.5=125 lattice points are defined. A three-dimensional LUT for defining the correspondency between an input color and an output color at each lattice point is prepared. If an input color to be converted corresponds to a lattice point, an output color is derived according to the definitions on the table. If there is no lattice point corresponding to the input color to be converted, an interpolation value P in a space defined by input data items A to H of eight points arranged discretely to surround the position of the input color is obtained. Based on the obtained interpolation value, an output color is obtained. The color transformation technique using the look-up table method is disclosed in, e.g. U.S. Pat. No. 4,275,413. A prism interpolation method, which will be described later, is an interpolation method used in the look-up table method accompanied with an interpolation.
As has been described above, color evaluation or color matching of a printed matter is normally performed under specific illuminant. In this case, by making use of the neural network or look-up table method, color transformation for color matching can be performed exactly. On the other hand, at present, there is a demand that exact color matching be performed so that the color of a printed matter may be the same even if observation light is changed. However, a correction technique for such color matching has not yet been developed.